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The Multispectral Sun

Your eyes tell you that the Sun obviously delivers energy to Earth in the form of visible light. If you think about it a bit, especially in terms of the choices you make about UV-A and UV-B protection when you shop for sunscreen or sunglasses, you'll also realize that you know that the Sun also bathes our planet in ultraviolet "light" or radiation. The Sun, in fact, emits radiation across most of the electromagnetic spectrum... from high-energy X-rays to ultra-long wavelength radio waves. Let's take a look now at this multispectral Sun and the energy it emits. Later in this week we'll see what happens to these different types of energy when they reach Earth. Electromagnetic radiation from the Sun is the main source of energy that drives Earth's climate system.

Review: The Electromagnetic Spectrum

Before we look at the multispectral Sun, let's review some key ideas about the electromagnetic (EM) spectrum. If you are very familiar with the EM spectrum (most chemistry and physics teachers are), feel free to skim this section. If this is an area that you are less comfortable with, we've provided some links to further readings that you might want to browse.

Visible light is the most familiar form of a type of energy called electromagnetic radiation. Light is actually rather strange stuff. In some ways it behaves like a stream of particles (which we call "photons"), and in other ways it acts like a series of waves. Light travels at a finite, albeit absurdly high, speed of 299,792 km/sec (186,282 miles per second); a beam of light could circle the globe more than seven times in a single second!

Light comes in different colors, spread across the rainbow of hues we call the visible spectrum. Each color corresponds to waves with different wavelengths. Red waves are the longest, purple the shortest. Since all colors of light (and all types of EM waves, for that matter) travel at the same speed, wavelength is inversely proportional to frequency (frequency is how often the "crest" of a wave passes by a given location). Long wavelength red waves have low frequencies; short wavelength purple waves have high frequencies. Different colors of light also carry differing amounts of energy. High frequency, short wavelength purple light carries the most energy; low frequency, long wavelength red light carries the least energy.

The visible light spectrum ranges from short-wavelength violet to long-wavelength red. Photons of light from violet end of the spectrum have the highest energies and the highest frequencies, while red photons have lower energies and lower frequencies. Beyond the range of our vision are the longer wavelengths of the infrared and the shorter wavelengths of the ultraviolet regions of the electromagnetic spectrum. Credit: Artwork by Randy Russell.

Visible light is not, however, the whole story by any means. Visible light is but one small segment of the entire electromagnetic spectrum. Waves that have wavelengths slightly shorter than purple light, and thus have slightly higher frequencies and higher energy levels, are called ultraviolet ("beyond violet", from the Latin ultra = "beyond") or UV "light" or radiation. We cannot see UV "light", though some animals, like honeybees, can. Likewise, just beyond the other end of the visible spectrum lie waves with wavelengths slightly longer that red light waves. These waves, which have even lower frequencies and carry somewhat less energy than red light, are called infrared ("below red", from the Latin infra = "below") or IR "light" waves.

Of course, IR and UV and visible light are not the whole story either. Beyond the UV portion of the spectrum lie the still shorter waves (with higher frequencies and greater energies) of X-rays. Beyond X-rays lie the extremely short wavelength gamma rays, which have exceptionally high energies and frequencies. Moving in the other direction, out beyond the infrared portion of the EM spectrum, we find various types of radio waves. All radio waves have longer wavelengths than infrared waves, and thus carry less energy and have lower frequencies. Microwaves (yes, the kind employed in microwave ovens) are relatively short wavelength (and thus relatively high energy) radio waves. Back when broadcast television signals were common, the waves that carried TV signals to our antennas were a type of radio wave. Of course, radio signals, both AM and FM, are also carried by radio waves.

This depiction of electromagnetic spectrum shows several objects with size scales comparable to the wavelengths of the waves of different types of electromagnetic radiation. Note that the range of wavelengths vary by many orders of magnitude, while the waves shown in this "cartoon" do not. For example, visible light waves are typically 100 time shorter than infrared waves, not just slightly shorter as depicted pictorially. Credit: Image courtesy of NASA's "Living With a Star" program and the Center for Science Education at Space Sciences Laboratory, University of California at Berkeley.

The Sun's Emissions of Electromagnetic Radiation

The Sun emits EM radiation across most of the electromagnetic spectrum. Although the Sun produces gamma rays as a result of the nuclear fusion process (see the diagram of the proton-proton chain on "The Solar Furnace" reading page), these super high energy photons are converted to lower energy photons before they reach the Sun's surface and are emitted out into space. So the Sun doesn't give off any gamma rays to speak of. The Sun does, however, emit x-rays, UV, light (of course!), IR, and even radio waves.

The peak of the Sun's energy output is actually in the visible light range. This may seem surprising at first, since the visible region of the spectrum spans a fairly narrow range. And what a coincidence, that sunlight should be brightest in the range our eyes are capable of seeing! Coincidence? Perhaps not! Imagine that our species had "grown up" on a planet orbiting a star that gave off most of its energy in the ultraviolet region of the spectrum. Presumably, we would have evolved eyes that could see UV "light", for light of that sort is what would be most brightly illuminating our planet's landscapes. The same sort of reasoning would apply to species that evolved on planets orbiting stars that emit most of their energy in the infrared; they would most likely evolve to have IR sensitive eyes. So it seems that our eyes are tuned to the radiation that our star most abundantly emits.

The graph below shows a simplified representation of the energy emissions of the Sun versus the wavelengths of those emissions. The y-axis shows the relative amount of energy emitted at a given wavelength (as compared to a value of "1" for visible light). The x-axis represents different wavelengths of EM radiation. Note that the scale of the y-axis is logarithmic; each tick mark represents a hundred-fold increase in amount of energy as you move upward.

This graph shows (approximately) the distribution of the EM energy emitted by the Sun vs. the wavelength of that energy. Long-wavelength radio waves are to the right, short wavelength X-rays are to the left. The units of energy along the vertical axis are relative to the peak in the Sun's EM energy output in the visible light part of the spectrum, which is arbitrarily given the value of "1". Note that the vertical scale is logarithmic, so that each tick mark represents a hundred-fold increase/decrease in energy. Credit: Image courtesy of the COMET program and the High Altitude Observatory at NCAR (the National Center for Atmospheric Research).

Physicists use a concept called a "blackbody radiator" to explain how hot objects emit EM radiation of different wavelengths. Although a blackbody radiator is a mental construct, not a real object, many real objects behave almost like a blackbody radiator. As an example, imagine a piece of iron that is heated in a furnace. At first, when the iron is not especially hot, it will not glow at all; but if you put your hand near it, you could feel the heat it was giving off. At this relatively low temperature the iron radiates most of its energy in the IR portion of the spectrum, which we cannot see but which we can feel as heat. As the iron gets warmer, it begins to glow deep red; the peak of its radiation has just moved into the lowest energy, longest wavelength portion of the visible spectrum just above the infrared. As the iron grows hotter, its glow becomes orange and then yellow, as its peak emissions creep up the spectrum to higher energies and shorter wavelengths.

So what does this have to do with the Sun? An iron bar made of solid metal and a giant ball of gas-like plasma made of hydrogen and helium don't seem very similar, and you might not expect them to behave at all alike. Nevertheless, the visible "surface" of the Sun behaves pretty much like an idealized blackbody radiator. Recall that the temperature of the photosphere is around 5,800 K. The graph below shows the theoretical radiation curve for a blackbody radiator with a temperature of 5,800 K. Notice that over much of the EM spectrum the Sun looks very much like a blackbody radiator. Notice also that the Sun emits many more high-energy photons in the UV and X-ray regions of the spectrum than a blackbody radiator would. So what does all of this mean?

This graph shows the distribution of the EM energy emitted by a "blackbody radiator" having a temperature of the surface of the Sun (approximately 5,800 kelvin). Compare this with the energy distribution of the Sun shown above. Note how the two curves are virtually identical from the near ultraviolet to the radio wave regions, but are quite different in the far ultraviolet and X-ray regimes. The radiation emitted from the Sun's atmosphere causes the Sun to behave differently than an ideal blackbody radiator. Credit: Image courtesy of the COMET program and the High Altitude Observatory at NCAR (the National Center for Atmospheric Research).

The Sun's photosphere behaves pretty much like a blackbody radiator. If we look at the Sun in visible or IR light, we will pretty much be observing the photosphere. However, the Sun emits more high energy UV and X-rays than a blackbody radiator would. These high energy photons are mainly emitted from the Sun's atmosphere. Recall how the temperature of the Sun gradually declined from 15 million K at the core to 5,800 K at the photosphere, but then surprisingly rose again to 3 million K in the Sun's outer atmosphere (the corona). The high temperatures of the solar atmosphere, along with explosive phenomena like solar flares that further energize this region of the Sun, generate high energy UV and X-ray photons. Furthermore, different wavelengths of UV and X-ray emissions come from different heights in the solar atmosphere, so we can view different levels of the Sun's atmosphere by looking at specific wavelengths of these UV and X-ray emissions. Let's examine some of these multispectral views of the Sun now.

The Multispectral Sun

The Sun emits electromagnetic radiation at many wavelengths across the EM spectrum. These images show the Sun in the infrared, visible light, four different ultraviolet wavelengths, and in X-rays. Images taken in very narrow bands have the wavelengths of the associated waves noted (in nanometers). The photosphere is most prominent in the visible light images, while UV and X-ray views show details of the solar atmosphere. Note that almost all of these images are "false color" representations, since your eyes cannot see X-rays or ultraviolet or infrared "light".

This animation shows views of the Sun at various frequencies across the electromagnetic spectrum. Note how different features and regions of the Sun are visible in the different views. The visible light view shows the photosphere, including several sunspots. The infrared view shows the lower chromosphere immediately above the photosphere, where temperatures are still relatively cool. Most of the high energy photons that produce the UV and X-ray views come from higher up in the Sun's hot atmosphere. Notice how the areas of the atmosphere above sunspots tend to be especially bright in the X-ray and UV views. Sunspots are visible indicators of magnetic disturbances on the Sun that spawn high-energy phenomena such as solar flares and coronal mass ejections.

Most of the individual views portray the Sun as seen through a very narrow range of wavelengths; for example, the IR view is just a narrow band of infrared "light" with a wavelength around 1,083 nanometers (as opposed to the entire IR portion of the spectrum, which ranges across wavelengths from 750 nm to 1 mm), while the first UV image is centered around a wavelength of 30.4 nanometers. These narrow wavelength "windows" in the EM spectrum are actually the "fingerprints" of specific elements at specific temperatures. Hot gases and plasmas emit light (or UV radiation or X-rays) at very specific wavelengths, depending on the element involved and the temperature of that element. For example, the IR image with a wavelength of 1,083 nm is produced by atoms of helium indicative of temperatures of a few thousand kelvin. The 30.4 nm wavelength UV image is also produced by helium, but that helium has been ionized (stripped of one of its two electrons), which indicates that its temperature is somewhere in 60,000 to 80,000 kelvin range, and thus that it is somewhere around the boundary between the upper chromosphere and the hotter corona. The shorter wavelength, higher energy photons that produce the 19.5 nm wavelength UV image indicate an even hotter region higher up in the corona; they are emitted by iron (yes, the Sun has vaporized iron in it!) atoms that have had 11 of their electrons stripped away by temperatures around 1.5 million kelvin. The main upshot of all this is that different wavelength images allow us to see material at different temperatures on and above the Sun. In many cases, this means that each different wavelength image provides us with a view of material at a different height within the solar atmosphere. Also, certain features are especially prominent at different wavelengths.

"OK", you may say, "these are indeed pretty pictures... but what does all of this have to do with Earth's climate?". As we'll see in later pages, different wavelengths of EM radiation behave differently when they reach Earth's atmosphere. Fortunately for us, most of the high energy X-rays and ultraviolet radiation are absorbed by our atmosphere far above our heads, preventing them from frying us. They do, however, transfer their energy to the atmosphere at various levels, which has implications for our climate. Also, as we'll see in the very next reading, the amount of radiation emitted by the Sun in various wavelengths is not completely constant over time. Short term events like solar flares can dramatically alter the levels of X-ray and UV emissions from the Sun over the course of a few minutes. Multi-year cycles in solar activity only slightly alter the amount of visible light the Sun emits (to the tune of 0.1%), but can change the levels of X-ray and UV emissions by a hundred-fold.

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